This invention relates to a method and apparatus for measuring the organic contamination of an electronic circuit assembly, for example a printed circuit board.
The cleanliness of electronic circuit assemblies (ECA), such as printed circuit boards (PCB), is generally regarded as being critical to their functional reliability. Ionic and nonionic contamination on circuit boards are believed to contribute to premature failures of the circuit boards by allowing short circuits to develop.
In the manufacture of electronic circuit assemblies, ionic and nonionic contamination can accumulate after one or more steps of the process. Circuit board materials are plated, etched, handled by operators in assembly, coated with corrosive or potentially corrosive fluxes and finally soldered. Of course, there is cleaning at various steps along the way--for example after plating, etching and soldering--but each step, nevertheless, represents a potential source of ionic and nonionic contamination which may be carried over on surfaces of the finished circuit. It is therefore most important that a thorough cleaning of the surfaces be achieved after soldering. This is probably the last opportunity to remove ionic and nonionic contamination accumulating on the circuit in the manufacturing processes.
It is common to evaluate the level of ionic contamination on an assembled board by a solvent extraction test. The board to be tested is washed with an isopropanol/water mixture which is then subjected to an electrical conductivity/resistivity measurement from which an ionic concentration is calculated and expressed as a quantity of sodium chloride per unit area of board surface.
Until now there has been no simple quantitative analysis for the organic contaminants on a cleaned ECA. "Organic contaminants" herein includes ionic and nonionic contaminants which are not detectable by conductivity/resistivity measurements known for measuring ionic contamination. In order to better study the effectiveness of various solvents in removing the organic contaminants such as rosin flux from soldered boards, it is desired to provide a method and apparatus for determining the residual organic contaminants such as rosin flux on an ECA. It is further desired to provide a method and apparatus for substantially automatically and continuously determining the quantity of organic contaminants on an ECA. It is further desired to provide a method and apparatus for determining organic contaminants down to concentrations of about 1 microgram per square inch.